In the field of graphic arts, in order to improve the reproduction of continuous gradation images with halftone dot images or the reproduction of line images, image formation systems exhibiting ultrahigh-contrast (particularly, .gamma. is 10 or more) photographic characteristics are required.
An image formation system capable of obtaining ultrahigh-contrast photographic characteristics by development with a processing solution that has good storage stability has been demanded. To cope with this demand, as described in U.S. Pat. Nos. 4,166,742, 4,168,977, 4,221,857, 4,224,401, 4,243,739, 4,272,606, and 4,311,781, a system of forming an ultrahigh-contrast negative image having a .gamma. value exceeding 10 was proposed. In the system, a surface latent image-type silver halide photographic light-sensitive material, having added thereto a specific acylhydrazine compound, is processed with a developer containing 0.15 mol/l or more of a sulfurous acid preservative and having a pH of from 11.0 to 12.3. This new system is characterized in that silver iodobromide or silver chloroiodobromide can be used, though only silver chlorobromide having a high silver chloride content can be used in a conventional ultrahigh-contrast image formation system. Further, the new system is characterized in that a large amount of sulfurous acid preservative can be contained, and relatively good storage stability is achieved, though use of only a very small amount of sulfurous acid preservative is allowed in conventional lith developers.
In European Unexamined Patent Publication (EP) 0 208 514A, JP-A-61-223734 ("JP-A" means unexamined published Japanese patent application), and JP-A-63-46437, high-contrast photographic light-sensitive materials containing two types of silver halide grains, and further containing a hydrazine derivative, are described. These light-sensitive materials are processed with developers having a high pH of about 11.5. However, developers having a pH of 11 or more are prone to air oxidation, and they are unstable and not endurable in use or storage for a long period of time. In addition, since attention must be given to both handling and processing of a waste solution of the developers, a solution having a high pH is unfavorable from an ecological viewpoint. Rather, a means for subjecting to development a silver halide photographic light-sensitive material containing a hydrazine compound with a developer having a lower pH, is preferred in forming a high-contrast image.
On the other hand, when the pH of the developer is low, the contrast-increasing effect induced by the hydrazine derivative is so lowered that a high-contrast image cannot be obtained. In order to accelerate the contrast-increasing effect, development of hydrazine derivatives having higher activity and nucleation accelerators has been attempted. However, use of these compounds, on some occasion, deteriorates long-term storage stability of the light-sensitive material.
JP-A-4-331951 describes, in its claim, a high-contrast light-sensitive material that comprises a hydrazine derivative and silver halide grains that have been subjected to color sensitization in higher concentration of a dye per the surface area of a silver halide grain, than the other silver halide grains do. Further, British unexamined patent publication (GB-A) 9407599 describes, in its claim, a high-contrast light-sensitive material that comprises silver halide grains spectrally sensitized by an undetachable sensitizing dye, and silver halide grains not spectrally sensitized, and further a hydrazine derivative. In both cases, the spectrally sensitized light-sensitive grains and the spectrally unsensitized non-light-sensitive grains contribute to a silver image formed by image-wise exposure and development, due to the presence of the hydrazine derivative, thereby achieving a saving of the sensitizing dye and improvement of stain, while maintaining both high sensitivity and high density. However, the former case had a problem that, when photographic emulsions were left standing for a long time in a mixed state, the distribution of the dye in the mixed emulsions became homogeneous, so that a reduction in sensitivity was caused. Further, in the latter case, because the dye to be used was restricted to an undetachable dye, which was hardly detached from a photographic emulsion, satisfactory residual color could not be achieved. Further, these patent publications do not disclose a mixture of photographic emulsions that are different in the amount of a nitrogen-containing heterocyclic compound to be added thereto, respectively.